The rails of railroad tracks are conventionally laid down on transverse beams or crossties, which traditionally have been made of wood but lately are frequently also made of concrete, and which support the rails and with the aid of tie plates and other accessories prevent the rails from spreading apart. Although exact numbers are difficult to come by, a reasonable estimate would be that there are roughly one billion crossties currently in service on the North American continent (U.S. and Canada).
The physical condition of railroad crossties, which are subjected to high levels of dynamic loading by passing trains and are exposed to almost constantly changing ambient weather conditions and temperatures, tends to deteriorate with time. The deterioration may be either a degree of decay or the occurrence of some mechanical damage, and such deterioration may take place on the surface or in the interior of the ties. Thus, the condition of railroad crossties requires close monitoring, because their deterioration and resultant failure can lead to train derailments with possibly serious and even disastrous consequences to human lives, railroad equipment, public and private property, and the environment.
Heretofore, the traditional method of determining the condition of railroad crossties and whether or not they require replacement has involved periodic visual inspections of the ties carried out by the railroad companies' tie inspectors. These inspectors "walk the track" and visually examine each and every crosstie, looking for obvious signs of decay/deterioration and physical damage of the top surface of the crossties (for all practical purposes, the top of a tie other than the regions thereof beneath the rail-locating tie plates is the only accessible surface because the bottom rests on the track bed and the sides are usually covered by ballast). Ties deemed no longer suitable for continued service are marked and subsequently replaced.
However, because the covered top and side regions and the interiors of the ties are hidden from view, it is evident that the tie inspectors perform their inspections under less than the most favorable conditions, and that their decisions to designate crossties as unfit for service are frequently based on guesswork and erroneous evaluations. In fact, it is acknowledged in the railroad industry that up to about 20% of all crossties which are replaced are replaced unnecessarily, as proven by subsequent destructive examination (cross-sectioning) after removal. The principal reason why such an unwarranted economic loss is incurred is, of course, that the outside appearance of a crosstie is not always indicative of the tie's actual condition: for example, a tie may be superficially sound but decayed or cracked internally or at the bottom, and yet neither of these conditions would be visually detectable from above; on the other hand, a tie may show some superficial deterioration or damage, e.g., a split, at the top surface and yet be perfectly sound internally or at worst only minimally deteriorated and suitable for a number of additional years of service. Moreover, the economic cost of visual inspection is further heightened by virtue of the fact that, given the need for looking closely at each tie individually and for probing the same in case of doubt, a tie inspector even at best can inspect an average of about six miles of track per day.
Another known method of inspecting wood railroad crossties, which has been developed relatively recently, utilizes an instrument which is available under the designation "PANLOGGER" from Pandrol Limited of London, England. This instrument consists of a unit that receives, processes and stores information, and a probe which makes contact with the top surface of the tie to deliver thereto controlled impacts that vibrate the tie. In theory, the response is a measure of the vibration characteristics of the tie and, because those characteristics are affected by the presence or absence of decay or mechanical damage, of the need for replacing the tie. In practice, however, this method makes use of an analysis which is based on empirical data pertaining to the particular wood species (e.g., oak, maple, sweetgum, pine, etc.) of which the tie is made. Accuracy of the method thus is open to question, because empirical data valid for all possible cases are scarce and because, given the fact that there is a considerable variation in properties within each wood species, such empirical data are also unreliable.
Moreover, this method also results only in an evaluation of the average condition of the crosstie over its entire length and does not provide a determination of which part and how much of the tie is defective. The average condition of a crosstie, however, as will be readily apparent to those skilled in the art, is not a reliable measure of its suitability for continued service. Thus, a crosstie with very minor internal decay throughout its length, or a crosstie with minor degrees of decay under both tie plates but otherwise sound, or a crosstie with severe decay under one tie plate but otherwise sound throughout its length, all may provide the same average vibration response and thereby indicate the same average condition, yet the first one may be suitable for many more years of service, the second one may be suitable for at least several more years of service, and the third one may be totally unacceptable for any further service. Over and above this stands the fact that this method has been developed for use in conjunction with visual inspection, so that the previously mentioned time requirements for such an inspection are not ameliorated.
Another known device which has been suggested to be usable for testing concrete and wood structures, including wood railroad ties, is a low frequency ultrasonic tester available under the designation "JAMES V-METER" from James Instruments Inc. of Chicago, Ill. This device has been promoted as being capable of performing sonic pulse velocity measurements on coarse grain materials such as concrete and wood, among others, using pairs of various types of transducers (including both standard and rolling transducers), as being portable and of light weight, as being adapted for both A.C. and D.C. operation, and as providing direct digital readout of pulse transit time as well as digital printouts. However, the device is only adapted for manually performed individual spot testing procedures, which would be even slower than those performed with "PANLOGGER" instrument and so inefficient in a crosstie-testing environment as to be unacceptable, and which, apart from a time reading, provides no direct indication of the quality of the test object. Thus, the device cannot evaluate a length of railroad track including many crossties for overall integrity, nor can it provide an indication of the life expectancy of the individual crossties at a high rate.
Yet another known device which has been promoted as being suitable for testing objects of coarse grain structures by utilizing ultrasonic pulses applied through spaced probes or transducers to test objects of, among other materials, wood and concrete, is available under the designation "USL 33" from Krautkramer Branson of Lewistown, Pa. However, this device is also not designed to evaluate a length of railroad track including many crossties continuously at a high rate.